1. Field of the Invention
The present invention relates to an image forming apparatus for forming an image through laser exposure steps.
2. Description of the Related Art
An electrophotographic image forming apparatus forms an electrostatic latent image by applying laser light to a photoconductor uniformly charged. After that, toner is adhered to the electrostatic latent image to form a toner image, and the toner image thus formed is transferred onto a sheet of paper. In a laser exposure process during which the electrostatic latent image is formed, the laser light is emitted intermittently in accordance with image pulse signals depending on a dot pattern of the electrostatic latent image. At a time when a laser diode (LD) used as a laser light source is supplied with drive current greater than oscillation threshold current, the LD emits laser light. Therefore, in order to emit laser light intermittently, current control is so performed that the drive current is equal to or greater than the oscillation threshold current, or is smaller than the oscillation threshold current.
When the supply of drive current to the laser light source is started at an ON edge of a pulse for designating a light emission period in the image pulse signal, the start of laser light emission is delayed. This causes the laser light emission period to be shorter than the pulse width of the pulse of the image pulse signal. In short, pulse width reduction occurs. In order to perform laser exposure faithful to the image pulse signal, it is necessary to minimize the amount of pulse width reduction.
As for control over the amount of pulse width reduction, a technique is known in which bias current slightly smaller than oscillation threshold current is supplied to the laser light source, and thereby, the responsiveness of laser light emission is increased. An image forming apparatus is proposed which controls bias current in accordance with the result of detection of a luminous quantity in order to make the responsiveness constant independent of variation in responsiveness of the laser light source and of fluctuations in environmental temperature (Japanese Laid-open Patent Publication No. 2002-067376).
Another technique is disclosed for minimizing the amount of pulse width reduction. The technique is called “pulse width expansion” in which laser light emission is stopped after a predetermined lapse of time from an OFF edge of a pulse of the image pulse signal instead of stopping the laser light emission at an OFF edge of the pulse (Japanese Laid-open Patent Publication No. 2011-167898). According to the technique, laser light is emitted during a period of time corresponding to the pulse width of the expanded pulse at a time behind the pulse of the image pulse signal.
The amount of pulse width reduction corresponds to the sum of a time called an “oscillation delay time” and a time called a “rise time”. The oscillation delay time is a period of time from when increasing drive current in the laser light source is started (ON edge of a pulse of the image pulse signal) to when laser light emission is started. The rise time is a period of time from when the laser light emission is started to when the amount of light emission reaches a predetermined amount of light necessary for exposure.
The oscillation delay time significantly varies depending on operating ambient temperature. For low operating ambient temperatures, the oscillation delay time is short. For high operating ambient temperatures, the oscillation delay time is long. The oscillation delay time and the rise time in the laser light source tend to be longer as time advances.
The conventional technique for minimizing the amount of pulse width reduction by bias current cannot eliminate the pulse width reduction by an amount corresponding to the rise time. When the operating ambient temperature is low, the oscillation threshold current is small. In that case, the adjustment range of the bias current is small, which makes it impossible to minimize the amount of pulse width reduction.
According to the other conventional technique for minimizing the amount of pulse width reduction by pulse width expansion, the amount of pulse width expansion cannot be set minutely due to limitation on resolution determined based on the clock of a control circuit. Increasing the clock frequency probably causes noise.